1. Field of the Invention
The present invention relates to surgically implantable devices and, more particularly, to an apparatus and method for supporting the spine.
2. Description of the Related Art
Chronic lower back pain is a primary cause of lost work days in the United States. It is also a significant factor affecting both workforce productivity and health care expense. Therapeutic procedures for alleviating back pain range from conservative methods, e.g., with intermittent heat, rest, rehabilitative exercises, and medications to relieve pain, muscle spasm, and inflammation, to progressively more active and invasive surgical methods which may be indicated if these treatments are unsuccessful, including various spinal arthroplasties, and eventually even spinal arthrodesis, i.e., surgical fusion.
There are currently over 700,000 surgical procedures performed annually to treat lower back pain in the U.S. In 2004, it is conservatively estimated that there will be more than 200,000 lumbar fusions performed in the U.S., and more than 300,000 worldwide. These procedures represent approximately a $1 billion endeavor in an attempt to alleviate patients' pain. In addition, statistics show that only about 70% of these procedures performed will be successful in achieving this end.
Approximately 60% of spinal surgery takes place in the lumbar spine, and of that portion approximately 80% involves the lower lumbar vertebrae designated as the fourth lumbar vertebra (“L4”), the fifth lumbar vertebra (“L5”), and the first sacral vertebra (“S1”). Persistent low back pain is often attributable to degeneration of the disc between L5 and S1. Traditional, conservative methods of treatment include bed rest, pain and muscle relaxant medication, physical therapy or steroid injection. Upon failure of conservative therapy, spinal pain has traditionally been treated by surgical interventions. These surgeries have included spinal arthroplasty; arthrodesis, or fusion, which cause the vertebrae above and below the disc to grow solidly together and form a single, solid piece of bone. Yet, statistics show that only about 70% of these procedures performed will be successful in relieving pain.
There are multiple causes for a patient's lower back pain. The pain is frequently hypothesized to arise one or more of the following: bulging of the posterior annulus or PLL with subsequent nerve impingement; tears, fissures or cracks in the outer, innervated layers of the annulus; motion induced leakage of nuclear material through the annulus and subsequent irritation of surrounding tissue in response to the foreign body reaction, or facet pain. Generally, it is believed that 75% of cases are associated with degenerative disc disease. In cases of degenerative disc disease, the intervertebral disc of the spine suffers reduced mechanical functionality typically due to dehydration of the nucleus pulposus. Surgical procedures, such as spinal fusion and discectomy, may alleviate pain, but do not restore normal physiological disc function attributable to healthy anatomical form, i.e., intact disc structures such as the nucleus pulposus and annulus fibrosis, as described below.
The spinal column or backbone encloses the spinal cord and consists of 33 vertebrae superimposed upon one another in a series which provides a flexible supporting column for the trunk and head. The vertebrae cephalad (i.e., toward the head or superior) to the sacral vertebrae are separated by fibrocartilaginous intervertebral discs and are united by articular capsules and by ligaments. The uppermost seven vertebrae are referred to as the cervical vertebrae, and the next lower twelve vertebrae are referred to as the thoracic, or dorsal, vertebrae. The next lower succeeding five vertebrae below the thoracic vertebrae are referred to as the lumbar vertebrae and are designated L1-L5 in descending order. The next lower succeeding five vertebrae below the lumbar vertebrae are referred to as the sacral vertebrae and are numbered S1-S5 in descending order. The final four vertebrae below the sacral vertebrae are referred to as the coccygeal vertebrae. In adults, the five sacral vertebrae fuse to form a single bone referred to as the sacrum, and the four rudimentary coccyx vertebrae fuse to form another bone called the coccyx or commonly the “tail bone”. The number of vertebrae is sometimes increased by an additional vertebra in one region, and sometimes one may be absent in another region.
Each vertebra has a spinous process, which is a bony prominence behind the spinal cord that shields the spinal cord's nerve tissue. The vertebrae also have a strong bony “vertebral body” in front of the spinal cord to provide a platform suitable for weight-bearing. Each vertebral body has relatively strong, cortical bone layer comprising the exposed outside surface of the body, including the endplates, and weaker, cancellous bone comprising the center of the vertebral body. The bodies of successive lumbar, thoracic and cervical vertebrae articulate with one another and are separated by the intervertebral discs. Each intervertebral disc comprises a fibrous cartilage shell enclosing a central mass, the “nucleus pulposus” (or “nucleus” herein) that provides for cushioning and dampening of compressive forces to the spinal column. The shell enclosing the nucleus comprises cartilaginous endplates adhered to the opposed cortical bone endplates of the cephalad and caudal vertebral bodies and the “annulus fibrosis” (or “annulus” herein) comprising multiple layers of opposing collagen fibers running circumferentially around the nucleus pulposus and connecting the cartilaginous endplates. The natural, physiological nucleus is comprised of hydrophilic (water attracting) mucopolysacharides and fibrous strands of protein polymers. In a healthy adult spine, the nucleus is about 80% water by mass. The disc is a hydrostatic system. The nucleus acts as a confined fluid within the annulus. It converts compressive on the vertebral end plates (axial loads) into tension on the annulus fibers. The nucleus is relatively inelastic, but the annulus can bulge outward slightly to accommodate loads axially applied to the spinal motion segment.
The intervertebral discs are anterior to the spinal canal and located between the opposed end faces or endplates of a cephalad and a caudal vertebral bodies. The inferior articular processes articulate with the superior articular processes of the next succeeding vertebra in the caudal (i.e., toward the feet or inferior) direction. Several ligaments hold the vertebrae in position yet permit a limited degree of movement. The ligaments include the supraspinous, the interspinous, the anterior and the posterior longitudinal, and the ligamenta flava. The assembly of two vertebral bodies, the interposed, intervertebral, spinal disc and the attached ligaments, muscles and facet joints is referred to as a “spinal motion segment”. In essence, the spine is designed so that vertebrae “stacked” together can provide a movable support structure while also protecting the spinal cord's nervous tissue that extends down the spinal column from the brain.
The spine has defined range of motion. Its range of motion can be described in terms of degrees of motion. More particularly, the spines range of motion is typically described relative to translation and rotation about three orthogonal planes relative to an instantaneous center of rotation around the vertical axis of the spine. This can generally be broken down into six degrees of motion. These include flexion, extension, compression, rotation, lateral bending, and distraction.
Flexion and extension of the spine combine forward sliding and rotation of the vertebrae. The facet joints and the annulus resist the forward sliding. Rotation is resisted by the annulus; capsules of the facet joints; action of the back muscles, and passive tension generated by the thoracolumbar fascia. Extension is resisted by the facet joints, and secondarily by the annulus. The spine is typically resistant to injury if the force is only in pure flexion, as the combination of the facet joints and disc are intrinsically stable in this plane. While the spinal muscles can be injured during forceful flexion since they are important in controlling this motion, ensuing pain is not typically chronic. Extension is impaired by impaction of the facet joints and eventually the inferior articular process against the lamina. This can result in a cartilage injury of the facet joint; disruption of the facet capsule, and facet joint or pars interarticularis fracture.
Compression of the spine is due to body weight and loads applied to the spine. Body weight is a minor compressive load. The major compressive load on the spine is produced by the back muscles. As a person bends forward, the body weight plus an external load must be balanced by the force generated by the back muscles. That is, muscle loads balance gravitational loads so that the spine is in equilibrium, to preclude us from falling over. The external force is calculated by multiplying the load times the perpendicular distance of the load from the spine. In essence, the further the load is from the spine, the larger the compressive load is on the spine. Since the back muscles act close to the spine, they must exert large forces to balance the load. The force generated by the back muscles results in compression of spinal structures. Most of the compressive loads (˜80%) are sustained by the anterior column which includes the intervertebral discs and the vertebral bodies.
Compression injuries occur by two main mechanisms. It generally occurs by either axial loading by gravity or by muscle action. Gravitational injuries result from a fall onto the buttocks while muscular injuries result from severe exertion during pulling or lifting. A serious consequence of the injury is a fracture of the vertebral end plate. Since the end plate is critical to disc nutrition, an injury can change the biochemical and metabolic state of the disc. If the end plate heals, the disc may suffer no malice. However, if the end plate does not heal, the nucleus can undergo harmful changes. The nucleus loses its proteoglycans and thus, its water-binding capacity. The hydrostatic properties of the nucleus are compromised. Instead of sharing the load between the nucleus and the annulus, more load is transferred to the annulus. The annulus fibers then fail. In addition to annular tears, the layers of the annular separate (delaminate). The disc may collapse or it may maintain its height with progressive annular tearing. If the annulus is significantly weakened, there may be a rupture of the disc whereby the nuclear material migrates into the annulus or into the spinal canal causing nerve root compression.
Rotation of the spine is accomplished by the contraction of the abdominal muscles acting through the thorax and the thoracolumbar fascia. There are no primary muscles responsible for lumbar rotation. The facet joints and the collagen fibers of the annulus resist this rotation. In rotation, only 50% of the collagen fibers are in tension at any time, which renders the annulus susceptible to injury.
The spine is particularly susceptible to injury in a loading combination of rotation and flexion. Flexion pre-stresses the annular fibers. As the spine rotates, compression occurs on the facet joint surfaces of the joint opposite the rotation. Distraction occurs on the facet joint on the same side of the rotation. The center of rotation of the motion segment shifts from the back of the disc to the facet joint in compression. The disc shifts sideways and shear forces on the annular fibers are significant. Since the annular fibers are weak in this direction, they can tear. If the rotation continues, the facet joints can sustain cartilage injury, fracture, and capsular tears while the annulus can tear in several different ways. Any of these injuries can be a source of pain.
Lateral bending is a combination of lateral flexion and rotation through the annulus and facet joints.
Pure distraction rarely occurs and is usually a combination of tension and compression on the spinal joints depending on the direction of applied force. An example of a distraction force is therapeutic spinal traction to “unload” the spine. In the context of the present invention, the term distraction may refer procedurally to an elevation in height that increases the intervertebral disc space 860 resulting during or from introduction of a spinal implant. Temporary distraction will generally refer to elevation of disc height by an apparaus which is subsequently removed but wherein the elevation is retained intra-operatively, while the patient remains prone. Thus, an implant may be inserted into an elevated disc space 860 first created by another apparatus, and thereafter physical presence and dimensionality of the implanted apparatus would preserve the level of distraction.
Prior devices have typically not preserved, restored or otherwise managed these six ranges of motion. Accordingly, a need exists for apparatus and methods for preserving, restoring, and/or managing mobility of the spine.
As noted above, the nucleus pulposus that forms the center portion of the intervertebral disc consists of 80% water that is absorbed by the proteoglycans in a healthy adult spine. With aging, the nucleus becomes less fluid and more viscous and sometimes even dehydrates and contracts causing severe pain in many instances. This is sometimes referred to as “isolated disc resorption”. The intervertebral discs serve as “dampeners” between each vertebral body. They generally function to minimize the impact of movement on the spinal column. Intervertebral disc degeneration, which may be marked by a decrease in water content within the nucleus, can render the intervertebral discs ineffective in transferring loads to the annulus layers. In addition, the annulus tends to thicken, desiccate, and become more rigid with age. This decreases the ability of the annulus to elastically deform under load which can make it susceptible to fracturing or fissuring. One form of disc degeneration occurs when the annulus fissures or is torn. The fissure may or may not be accompanied by extrusion of nucleus material into and beyond the annulus. The fissure itself may be the sole morphological change, above and beyond generalized degenerative changes in the connective tissue of the disc, and disc fissures can nevertheless be painful and debilitating. Biochemicals contained within the nucleus are enabled to escape through the fissure and irritate nearby structures.
A fissure also may be associated with a herniation or rupture of the annulus causing the nucleus to bulge outward or extrude out through the fissure and impinge upon the spinal column or nerves. This is commonly termed a “ruptured” or “slipped” disc. Herniations may take a number of forms. With a contained disc herniation, the nucleus may work its way partly through the annulus but is still contained within the annulus or beneath the posterior longitudinal ligament, and there are no free nucleus fragments in the spinal canal. Nevertheless, even a contained disc herniation is problematic because the outward protrusion can press on the spinal cord or on spinal nerves causing sciatica.
Another disc problem occurs when the disc bulges outward circumferentially in all directions and not just in one location. This occurs when, over time, the disc weakens bulges outward and takes on a “roll” shape. Mechanical stiffness of the joint is reduced and the spinal motion segment may become unstable, shortening the spinal cord segment. As the disc “roll” extends beyond the normal circumference, the disc height may be compromised, and foramina with nerve roots are compressed causing pain. Current treatment methods other than spinal fusion for symptomatic disc rolls and herniated discs include “laminectomy” which involves the surgical exposure of the annulus and surgical excision of the symptomatic portion of the herniated disc followed by a relatively lengthy recuperation period. In addition, osteophytes may form on the outer surface of the disc roll and further encroach on the spinal canal and foramina through which nerves pass. The cephalad vertebra may eventually settle on top of the caudal vertebra. This condition is called “lumbar spondylosis”.
Various other surgical treatments that attempt to preserve the intervertebral spinal disc and to simply relieve pain include a “discectomy” or “disc decompression” to remove some or most of the interior nucleus thereby decompressing and decreasing outward pressure on the annulus. In less invasive microsurgical procedures known as “microlumbar discectomy” and “automated percutaneous lumbar discectomy”, the nucleus is removed by suction through a needle laterally extended through the annulus. Although these procedures are less invasive than open surgery, they nevertheless suffer the possibility of injury to the nerve root and dural sac, perineural scar formation, re-herniation of the site of the surgery, and instability due to excess bone removal. In addition, they generally involve the perforation of the annulus.
Although damaged discs and vertebral bodies can be identified with sophisticated diagnostic imaging, existing surgical interventions so extensive and clinical outcomes are not consistently satisfactory. Furthermore, patients undergoing such fusion surgery experience typically have significant complications and uncomfortable, prolonged convalescence. Surgical complications may include disc space 860 infection, nerve root injury, hematoma formation, instability of adjacent vertebrae, and disruption of muscles, tendons, and ligaments.
Any level of the spine can be affected by disc degeneration. When disc degeneration affects the spine of the neck, it is referred to as cervical disc disease, while when the mid-back is affected, the condition is referred to as thoracic disc disease. Disc degeneration that affects the lumbar spine causes pain localized to the low back and is sometimes common in older persons and known as lumbago Degenerative arthritis (osteoarthritis) of the facet joints is also a cause of localized lumbar pain that can be diagnosed via x-ray analysis.
Radiculopathy refers to nerve irritation caused by damage to the disc between the vertebrae. This occurs because of degeneration of the annulus fibrosis of the disc, or due to traumatic injury, or both. Weakening of the annulus may lead to disc bulging and herniation. With bulging and herniation, the nucleus pulposus can rupture through the annulus and abut the spinal cord or its nerves as they exit the bony spinal column. When disc herniation occurs, the rupture of the nucleus pulposus into or through the annulus may irritate adjacent nervous tissue, causing local pain, or discogenic pain, in the affected area.
The pain from degenerative disc or joint disease of the spine may be treated conservatively with intermittent heat, rest, rehabilitative exercises, and medications to relieve pain, muscle spasm, and inflammation. However, if these treatments are unsuccessful, progressively more active interventions may be necessary. These more active interventions may include spinal arthroplasty including prosthetic nucleus device implantation; annulus repair, and total disc replacement, and eventually, even spinal arthrodesis. The nature of the intervention performed depends on the overall status of the spine, and the age and health of the patient. In some cases, the intervention may include the removal of the herniated disc with laminotomy (a small hole in the bone of the spine surrounding the spinal cord), a laminectomy (removal of the bony wall), percutaneous discectomy (removal by needle technique through the skin), chemonucleolysis (various chemical disc-dissolving procedures), among other procedures.
When narrowing of the spaces in the spine results in compression of the nerve roots or spinal cord a condition known as spinal stenosis may occur. Spinal stenosis occurs when bony spurs or soft tissues, such as discs, impinge upon the spinal canal to compress the nerve roots or spinal cord. Spinal stenosis occurs most often in the lumbar spine, but also occurs in the cervical spine and less often in the thoracic spine. It is frequently caused by degeneration of the discs between the vertebrae due to osteoarthritis. Rheumatoid arthritis usually affects people at an earlier age than osteoarthritis does and is associated with inflammation and enlargement of the soft tissues of the joints. The portions of the vertebral column with the greatest mobility, i.e., the cervical spine, are often the portions most affected in people with rheumatoid arthritis. However, there are known non-arthritic causes of spinal stenosis. Some non-arthritic causes of spinal stenosis include tumors of the spine, trauma, Paget's disease of bone, and fluorosis.
Therapeutic procedures to alleviate pain are restore function are described in a progression of treatment from spinal arthroplasty to spinal arthrodesis. As used herein, spinal arthroplasty encompasses options for treating disc degeneration when arthrodesis is deemed too radical an intervention based on an assessment of the patient's age, degree of disc degeneration, and prognosis. Spinal arthrodesis, or fusion, involves a discectomy, i.e., surgical removal of the disc, followed by the subsequent immobilization of a spinal motion segment. A spinal motion segment is generally comprised of two adjacent vertebral bodies separated axially by a spinal disc. The procedure of discectomy and “fusion” of the vertebral bodies results in the two vertebrae effectively becoming one solid bone. Accordingly, the procedure terminates all motion at that joint in an attempt to eliminate or at least ameliorate discogenic pain. The benefit of fusion is pain relief and the down side is elimination of motion at the fused joint, which can hinder function. This surgical option is reserved for patients with advanced disc degeneration.
Several companies are pursuing the development of prosthesis for the human spine, intended to partly or completely replace a physiological disc. In individuals where the degree of degeneration has not progressed to destruction of the annulus, rather than a total artificial disc replacement, a preferred treatment option may be to replace or augment the nucleus pulposus. This augmentation may involve the deployment of a prosthetic disc nucleus. As noted previously, the normal nucleus is contained within the space bounded by the bony vertebrae above and below it and the annulus fibrosus, which circumferentially surrounds it. In this way the nucleus is completely encapsulated and sealed with the only communication to the body being a fluid exchange that takes place through the bone interface with the vertebrae, known as the endplates. The hydroscopic material comprising the physiological nucleus has an affinity for water which is sufficiently powerful to distract (i.e., elevate or “inflate”) the intervertebral disc space, despite the significant physiological loads that are carried across the disc in normal activities. These forces, which range from about 0.4× to about 1.8× body weight, generate local pressure well above normal blood pressure, and the nucleus and inner annulus tissue are, in fact, effectively avascular. In essence, the existence of the nucleus, as a cushion, and the annulus, as a flexible member, contributes to the range of motion in the normal disc.
As noted previously, some current devices are configured to form an artificial disc or an artificial nucleus. However, many of these devices are susceptible to movement between the vertebral bodies. Further, they may erode or degrade. In addition, they may extrude through the site of implantation in the annulus or otherwise migrate out of place. Some of these drawbacks relate to the fact that their deployment typically involves a virtually complete discectomy achieved by instruments introduced laterally through the patient's body to the disc site and manipulated to cut away or drill lateral holes through the disc and adjoining cortical bone. The endplates of the vertebral bodies, which comprise very hard cortical bone and help to give the vertebral bodies needed strength, are usually weakened or destroyed during the drilling. The vertebral endplates are special cartilage structures that surround the top and bottom of each vertebra and are in direct contact with the disc. They are important to the nutrition of the disc because they allow the passage of nutrients and water into the disc. If these structures are injured, it can lead to deterioration of the disc and altered disc function. Not only do the large laterally drilled hole or holes compromise the integrity of the vertebral bodies, but the spinal cord can be injured if they are drilled too posteriorly.
Alternatively, current devices are sometimes deployed through a surgically created or enlarged hole in the annulus. The annulus fibrosis consists of tough, thick collagen fibers. The collagen fibers which comprise the annulus fibrosis are arranged in concentric, alternating layers. Intra-layer orientation of these fibers is parallel, however, each alternating (i.e., interlayer) layers' collagen fibers are oriented obliquely (˜120′). This oblique orientation allows the annulus to resist forces in both vertical and horizontal directions. Axial compression of a disc results in increased pressure in the disc space. This pressure is transferred to the annulus in the form of loads (stresses) perpendicular to the wall of the annulus. With applied stress, these fibrous layers are put in tension and the angle from horizontal decreases to better resist the load, i.e., the annulus works to resist these perpendicular stresses by transferring the loads around the circumference of the annulus (Hoop Stress). Vertical tension resists bending and distraction (flexion and extension). Horizontal tension resists rotation and sliding (i.e., twisting). While the vertical components of the annulus' layers enable the disc to withstand forward and backward bending well, only half of the horizontal fibers of the annulus are engaged during a rotational movement. In general, the disc is more susceptible to injury during a twisting motion, deriving its primary protection during rotation from the posterior facet joints; however, this risk is even greater if and when the annulus is compromised.
Moreover, annulus disruption will remain post-operatively, and present a pathway for device extrusion and migration in addition to compromising the physiological biomechanics of the disc structure. Other devices, in an attempt to provide sufficient mechanical integrity to withstand the stresses to which they will be subjected, are configured to be so firm, stiff, and inflexible that they tend to erode the bone or become imbedded, over time, in the vertebral bodies, a phenomenon known as “subsidence”, sometimes also termed “telescoping”. The result of subsidence is that the effective length of the vertebral column is shortened, which can subsequently cause damage to the nerve root and nerves that pass between the two adjacent vertebrae.